1. Field of the Invention
The invention relates to a display device, particularly to a display device having a DA converter for converting a digital image signal to an analog image signal.
2. Description of the Related Art
There has been a great demand on the market for portable display devices such as a portable TV and a portable telephone. All these devices need a small, lightweight and low power consumption display device, and development efforts have been made accordingly.
FIG. 10 is a circuit diagram of a pixel of a conventional liquid crystal display device. A gate signal line 51 and a drain signal line 61 are formed on an insulating substrate (not shown), crossing each other. In a peripheral of the crossing, a pixel selecting thin film transistor 72 is provided, being connected to both the signal lines 51 and 61. Hereinafter, a thin film transistor is referred to as “TFT”. A source 11s of the pixel selecting TFT 72 is connected to a display electrode 80 of a liquid crystal 21.
A storage capacitor 85 for retaining a voltage of the display electrode 80 for one field period is provided. One terminal 86 of the storage capacitor 85 is connected to the source 11s of the pixel selecting TFT 72, and another terminal 87 is applied with an electric potential common to all the pixels.
Here, when a scanning signal (high level) is applied to the gate signal line 51, the pixel selecting TFT 72 turns on, and an analog image signal is transferred to the display electrode 80 from the drain signal line 61 and retained in the storage capacitor 85. The image signal voltage applied to the display electrode 80 is applied to the liquid crystal 21. The liquid crystal 21 aligns itself in response to the voltage, resulting in a liquid crystal display. Accordingly, a liquid crystal display of either a motion picture or a still picture is obtained.
The analog image signal inputted to the drain signal line 61 is obtained by converting an inputted digital image signal to an analog image signal by a DA (digital-to-analog) converter. Conventionally, in a liquid crystal display device which is integrated with the DA converter in a display panel, the DA converter is placed adjacent a driver circuit in a peripheral of the pixel.
In the conventional liquid crystal display device, since the DA converter is placed adjacent the driver circuit, peripheral circuits of the pixel are complicated, providing a problem of increasing a frame area of the display panel. Particularly, when a gray-scale voltage is inputted from an outside, the number of the terminals increases by the number of the gray-scale signals.
Furthermore, since the analog data converted by the DA converter is written in the pixel through the pixel selecting TFT 72, a voltage of a sum of the maximum amplitude voltage and Vth, or higher needs to be supplied as a scanning signal. Here, Vth is a threshold voltage of the pixel selecting TFT 72. Therefore, it is difficult to lower the voltage and the power consumption of the liquid crystal display device.